The present invention relates to an optical transmission device and an optical transmission system. More particularly, the present invention relates to an optical transmission device and an optical transmission system suitable for low-noise transmission.
In an attempt to satisfy a requirement of lowering the cost for an optical transmission system, a wavelength divisional multiplexing optical transmission system, which transmits different wavelengths of signal lights in one single optical transmission fiber, has been considered. In particular, a bi-directional optical transmission system, which transmits different wavelengths of light signals in a single optical transmission fiber bi-directionally, is suitable when exchange of information is needed interactively between the two connected stations. Under such a technical background, it has become more important to provide an optical amplifier applicable to a bi-directional optical transmission system.
Japanese Patent Laid-open No. Hei 6-85369 describes as a conventional apparatus an optical amplifier. The optical amplifier includes apparatus for multiplexing or demultiplexing signal lights in a forward or a reverse direction toward both ends of a doped fiber. The optical amplifier is capable of sharing the use of one optical amplifying medium and one optical pumping source in the forward or the reverse direction, and is applicable to a bi-directional optical transmission system, the constitution of which is simple.
Japanese Patent Laid-open No. Hei 9-98136 describes another example of an optical amplifier which is capable of controlling the individual wavelength output even if there occur variations in signal wavelength multiplexity.
The optical amplifiers disclosed by the above-identified Japanese patent applications have various disadvantages in their practical use as described below. It is generally known that, in a one-directional optical amplifier having one doped fiber, a signal light input loss at a step previous to the doped fiber is attributed to a degradation in the S/N ratio.
"Optical Amplifiers and Their Application" (Ohm Publishing, May, 1992, pp 5-3[1]), describes that it is essential to combine an optical isolator at the front of doped fiber for suppressing reflexed amplified spontaneous emission (ASE). The optical isolator is not the only optical component which is inserted at the front of doped fiber. Generally, a transmission equipment requires a wavelength demultiplexer for an optical surveillance signal, a optical coupler for an optical signal monitor and a wavelength multiplexer for a pumping light. All of these optical components have loses. Further, the noise figure of Erbium doped fiber having a length of 20-30 m is not negligible. Where the noise figure is defined by the ratio of the S/N ratio on the input side and the S/N ratio on the output side.
The optical signal which is attenuated in the transmission path also suffers losses due to the optical components. The optical signal is amplified in the EDF of which a noise figure is large. The above-described transmission equipment cannot achieve a noise figure less than 6 dB.
When a non-regenerative multiple amplifying transmission is performed using k units of optical amplifiers, the S/N ratio degradation amount increases in proportion to the step number k. Accordingly, in an actual optical transmission system in which there exists an upper limit in the total S/N ratio degradation amount, the repeating step number decreases as the S/N ratio degradation amount in the optical amplifiers increases. This eventually shortens the light transmission distance.
For example, when setting optical amplifiers, the S/N ratio degradation amount of same are 4 dB, and the S/N ratio degradation amount of others are 6 dB at intervals of 80 km. Under a requirement that the total S/N ratio deterioration amount can not be more than 12 dB, a total S/N ratio degradation amount of the 4 dB optical amplifiers becomes 12 dB when three steps are repeated, and the total S/N ratio degradation amount of the 6 dB optical amplifiers becomes 12 dB when two steps are repeated. Thus, when the 4 dB optical amplifiers are used in three repeated steps it is possible, thus making it possible to transmit a signal light for 240 km. Whereas, when the 6 dB optical amplifiers are used in two repeated steps it is possible to transmit a signal light for 160 km.